Signals from cellphone masts can be used to track aircraft, monitor traffic congestion and spot speeding motorists without tipping them off that they are being watched.

The radar-like system, which is still being developed, has provoked media reports of the start of a huge extension of Big Brother-style surveillance - privacy campaigners have complained that it could be used to track individual people. But radar experts say such fears are unfounded.

Conventional radar works by transmitting a signal, listening for the reflection and using the time taken for the round trip to work out the object's distance. More sophisticated systems can work out the object's speed from characteristic changes to the signal's frequency, known as Doppler shifts. But such radar systems are expensive, and the signals they send out are easy to detect.

An alternative technique, called passive radar, gets round these problems. Instead of broadcasting its own signal, a passive radar system listens in to the cacophony of radio signals in the environment and monitors the way moving objects change them. The US defence company Lockheed Martin is developing a system called Silent Sentry which exploits the signals from radio and television masts to spot aircraft and ships (New Scientist print edition, 4 December 1999).

Better accuracy

Now two British companies, Hampshire-based Roke Manor Research and the aerospace giant BAe Systems, have done the same thing with signals from cellphone masts. They say their system, known as Celldar, short for cellphone radar, can achieve better accuracy because cellphone masts are far more widespread than television and radio transmitters.

Celldar works out the position of objects in the area by comparing the signals reflected from them with those it receives directly from a base station, whose positions are known. From the Doppler shift in the signal it can also calculate the target object's speed.

Celldar has a number of advantages over conventional radar, says David Salter, a member of the team at Roke Manor Research. "The expensive part in most radar systems is the transmitter, because of the high power requirements." Because Celldar devices do not need their own transmitter, they can be made cheaper, smaller and more portable.

Roke Manor Research is currently testing a prototype system, and says it will be two to three years before a fully operational Celldar goes on sale.

Stealthy shadows

Because the system is passive, drivers will have no way of telling whether they are being monitored. It is this characteristic that makes passive systems so attractive to the military, says David Bebbington, a radar expert at the University of Essex in Colchester, UK.

Another advantage of passive systems is their ability to spot "stealthy" aircraft and ships, which are designed to fool conventional radar systems by absorbing signals or reflecting them away from the source. To passive radar, these objects show up as shadows that can be spotted.

Civil liberties groups are concerned that the system could be adapted or combined with other technologies to produce a device for tracking people. "I can see profoundly worrying aspects to the technology," says Simon Davies, director of Privacy International in London.

A document on Roke Manor Research's own website has fuelled speculation that the technology could be used in this way, stating that it "can detect vehicles and even human beings at militarily useful ranges".

But Bebbington points out that Celldar will be virtually useless for following individuals because its resolution is simply not good enough. And Roke Manor Research now says the information on its website will be removed.